Thin-film electroluminescent (TFEL) display panels are becoming a popular device for providing real time visual representations of information. A TFEL display panel comprises an active layer of material that is sandwiched between two sets of electrodes that are typically perpendicular with respect to each other. When an electromotive potential, a voltage, is established between the two electrodes across the active layer, the active layer at that location becomes electrically excited and emits photons, or light, which is observed by a viewer. By selectively establishing voltages between the electrodes at various locations across the active layer, a TFEL display panel can be used to generate almost any type of visual representation, including images, graphics, and textual material.
TFEL display panels, and monitors that incorporate them, have a number of advantages over other types of video displays. TFEL display panels are significantly more compact, lightweight, rugged, and consume less power than cathode ray tubes which are large vacuum tubes that have interior surfaces with a layer of light-emitting material that requires significant voltages to emit light. TFEL display panels consume much less power than light-emitting diode (LED) displays which are formed from a large number of discrete LEDs that collectively consume so much power that they are unsuitable for large screen displays. Still another advantage of TFEL display panels is that they have wide viewing angles. In other words, the displayed image readily can be observed even when the viewer's perspective is significantly offset from being perpendicular to the face of the display. A further advantage of TFEL display panels is that they are readable in almost all light conditions, i.e., conditions of both very low and very high ambient light. These features make TFEL display panels more advantageous than other well-known displays such as liquid crystal displays (LCDs) that have limited viewing angles and require backlighting when only low ambient light is available.
Moreover, unlike other displays such as LEDs and LCDs, TFEL displays can readily be provided with an adjustable brightness control that allows the viewer to set display brightness to allow for changing ambient conditions or for individual preference. Accordingly, because of their advantages, there has been increasing interest in using TFEL display panels for avionics, automotive electronics, and other applications where it is desirable to provide the viewer with real time visual depictions of information that change rapidly, where the space for providing a display is minimal, the power available for driving a display is limited, and the ambient light is either not optimal and/or subject to change from optimal to that which is less.
Despite the advantages of TFEL display panels, there are still some limitations with them that have inhibited their application for many uses. These limitations have been associated with the drive voltages that are applied to TFEL display panels to cause them to emit light. These problems stem from the fact that the amount of light emitted, or brightness, at any one given point on a TFEL display, and at any given instant, is directly related to the drive voltages. The drive voltages necessary to cause a given amount of light to be emitted may vary from display to display, and may fluctuate in a single display panel over short periods of time as ambient conditions such as temperature change and over extended periods of use as the components forming the display change as an inherent part of the aging process.
The different drive voltage requirements of individual TFEL display panels can make it unduly troublesome to provide a TFEL monitor with a new display panel as may be required for repair or replacement purposes. Difficulties arise because each time a monitor is provided with an new or replacement display panel, significant amounts of time may be spent calibrating the monitor power supply that supplies the drive voltages so that when the voltages are applied, the display panel emits the desired brightness of light. Calibrating the monitor power supply is often further complicated because the calibration point, the point where the drive voltage adjustments to the power supply are made, is often located at a position in the monitor that is difficult to access.
Brightness-per-applied drive voltage fluctuations of a single TFEL display panel can adversely affect the operation of the monitor it is used with. As the display voltage necessary to emit a specific brightness changes, the display panel may either emit light when it is not supposed to, or not emit light when it should. Moreover, a viewer-set brightness control operates by controlling the level of the drive voltages applied to the TFEL display panel. Brightness-per-applied drive voltage fluctuations can radically alter the degree of brightness changes, as the display voltage is adjusted through changes in the brightness control setting. Thus, when the brightness control is set, there may be uneven variations in observed brightness levels that are both disconcerting and unsatisfactory to the viewer.
Other difficulties are associated with some TFEL display panels that are provided with a number of different voltages in order to cause the desired emission of light. One such type of TFEL display panel is an AC-TFEL display that emits light during the time the voltages are changing across the electrodes. A popular way of applying enough voltage to these display panels is to apply the necessary voltages twice, in short succession from each other. During the first write period, or frame, a positive write voltage is applied to one of the electrodes as part of the display voltage necessary to emit light. A short time thereafter, during a succeeding frame, a negative write voltage is applied as part of the necessary drive voltage. In most symmetrically driven TFEL display panels, the write voltages are alternately applied to all electrodes on one side of the active layer. The actual emission of light is controlled by selectively applying a modulation voltage to the opposite electrodes that, depending on the polarity of the applied write voltage, can or cannot establish a voltage across the active layer that results in the emission of visible light.
Problems arise with TFEL displays, including symmetrically driven TFEL displays, because, over time, the repeated applications of one of the drive voltages, in symmetrically driven displays the write voltages, may cause an electric potential or charge to become established in the components, including the active layer, forming the TFEL display panel. As a result of this charge formation, latent images of representations frequently represented on the display panel may appear. Latent images are typically of frames, textual information, or symbols that are repetitively produced on the display panel. Latent images can be "positive" images, faint outlines of images that always appear on the display panel even when they are not specifically being produced; they can also be "negative" images or shadows that should be fully illuminated but instead appear noticeably dimmer than adjacent illuminated regions of the display. Whatever type of latent image forms, it can detract from the image presented on the display panel, and significantly impair or disrupt the viewer's ability to perceive the intended image.